1. Field of the Invention
The present invention relates to a superconductor device. In particular, the superconductor device has an operating region made of superconductor material and an operating region made of either semiconductor or metallic material.
2. Description of the Prior Art
A superconductor transistor has been known as a superconductor device having a three-terminal structure (including a device comprising a single element and a device comprising an integrated circuit). The superconductor transistor comprises a semiconductor bipolar transistor having a base region made of superconductor material. In a semiconductor bipolar transistor, the resistance of the base region is critical in determining an operation speed, power consumption, and heat generation. Reduced resistance in the base region will achieve a faster operation speed, reduced power consumption, and lower heat generation. In order to achieve the above, superconductor material is employed for a base region in a superconductor transistor so as to solve technical problems.
As a typical emitter-base structure for a superconductor transistor, a Normal metal Insulator Superconductor (NIS) stacked structure has been known, and a great deal of studies have been made in this direction. In the NIS stacked structure, metallic material, insulator material, and superconductor material are successively stacked, thereby constituting an emitter region, a tunneling barrier region, and a base region, respectively. The collector region of a superconductor transistor employing the NIS stacked structure is made of semiconductor material. This semiconductor material also serves as a substrate of the superconductor transistor.
The base region may be made of BaKBiO-associated superconductor material. Because BaKBiO-associated superconductor material can be formed into a film (hereinafter referred to as a BaKBiO film) at a relatively low temperature, the semiconductor collector region (a semiconductor substrate), on which a BaKBiO film is formed, is not thermally damaged. That is, the use of BaKBiO-associated superconductor material enables a stacked structure using semiconductor material.
The semiconductor collector region may be made of an SrTiO.sub.3 crystal substrate (a single crystal substrate) doped with Nb. An SrTiO.sub.3 crystal substrate has a perovskite crystal structure, which is similar to the crystal structure of BaKBiO-associated superconductor material, and a crystal lattice interval ("a" axial length) of an SrTiO.sub.3 crystal substrate is approximate to the crystal lattice interval ("a" axial length) of BaKBiO-associated superconductor material. Thus, it is possible to form a BaKBio film on the surface of an SrTiO.sub.3 crystal substrate.
In the superconductor transistor employing the foregoing NIS stacked structure, a Schottky barrier region is formed on the collector region side between the base region (BaKBio-associated superconductor) and the collector region (a SrTiO.sub.3 crystal substrate), starting at the phase boundary between the two regions. Based on the carriers injected from the emitter region, carriers are transmitted from the base region to the collector region, passing through the Schottky barrier region. Further, based on the carriers injected from the base region to the collector region, collector current accordingly flows. A transmission efficiency of the carriers from the base region to the collector region can be effectively increased by setting a lower and shorter barrier in the Schottky barrier region.
In the above superconductor transistor, however, in cases where a lower and shorter barrier is set in a Schottky barrier region for the purpose of increasing a transmission efficiency, increased number of carriers leak from the base region to the collector region, passing through the Schottky barrier region. As a result, current-voltage characteristics are degraded in a superconductor transistor as leakage current is added to the base-collector current.